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Chapter 18
Gas-Phase Chemistry of First-Row Transition Metal Ions with Nitrogen-Containing Compounds Theoretical and Experimental Investigations
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A. Mavridis , K. Kunze , J. F. Harrison , and J. Allison 1
Chemistry Department, University of Athens, 13a Navarinou Street, Athens 10680 Greece Department of Chemistry, Michigan State University, East Lansing, MI 48824 2
The r i c h gas phase chemistry of f i r s t row t r a n s i t i o n metal (+1) ions with ammonia, and organic compounds (R-X where X=NH , CN, NO ) is discussed. Ongoing theoretical investigations into the Sc /NH system are presented, which provide some insights into the bonding and energetics of a variety of ΜΝΗ+ complexes. 2
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T h e r e a r e a number o f mass s p e c t r o m e t r i c t e c h n i q u e s t h a t have been d e v e l o p e d f o r t h e study o f t h e low p r e s s u r e g a s phase r e a c t i o n s o f i o n i c s p e c i e s w i t h organic molecules. The e a r l i e s t e x p e r i m e n t s involving ion/molecule reactions involved chemical ionization mass s p e c t r o m e t r y , and t h e most r e c e n t u t i l i z e i o n beam and F o u r i e r t r a n s f o r m i o n c y c l o t r o n r e s o n a n c e m e t h o d s . The earliest s t u d i e s were p r e d o m i n a n t l y organic i n nature. More r e c e n t l y , t h e s e m e t h o d s h a v e b e e n u s e d t o s t u d y o r g a n o m e t a l l i c c h e m i s t r y i n t h e gas phase. The various ionization techniques available i n mass spectrometry a l l o w f o r the g e n e r a t i o n o f unique gas phase s p e c i e s i n c l u d i n g bare t r a n s i t i o n metal ions ( s u c h a s Co*, N i ) and l i g a t e d s p e c i e s ( s u c h a s C o C O , CoNO , N i P F , N i C H ) . Their chemistry with small m o l e c u l e s and a v a r i e t y o f l a r g e r m o l e c u l e s c o n t a i n i n g t h e f u n c t i o n a l groups o f o r g a n i c c h e m i s t r y h a s been e x t e n s i v e l y s t u d i e d i n t h e p a s t 15 y e a r s (1) . The e a r l i e r s t u d i e s were l a r g e l y m e c h a n i s t i c i n n a t u r e , t o g a i n a n u n d e r s t a n d i n g o f how p r o d u c t i o n s w e r e f o r m e d . R e c e n t l y , c o m b i n e d e x p e r i m e n t a l and t h e o r e t i c a l e f f o r t s have provided important insights into how t h e s e reactions occur. These insights will become, we believe, the basis for a fresh evaluation o f the r e a c t i v i t y o f i n o r g a n i c and o r g a n o m e t a l l i c s p e c i e s t h a t a r e s t u d i e d and u t i l i z e d i n c o n d e n s e d p h a s e s . +
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0097-6156/90/0428-O263$06.00A) © 1990 American Chemical Society In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS
B o t h p o l a r a n d n o n p o l a r o r g a n i c compounds e x h i b i t a r i c h chemistry with bare t r a n s i t i o n metal i o n s ( l ) . S m a l l p o l a r compounds r e a c t w i t h i o n s s u c h a s F e a n d Co , i n a s i n g l e , b i m o l e c u l a r step t o form a metalo l e f i n c o m p l e x , r e a c t i o n (1) . +
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Co
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+ C H X 3
-> C o C H
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+ HX
(1)
S u c h r e a c t i o n s h a v e b e e n r e p o r t e d f o r X=C1, B r , OH, SH, OR a n d e v e n f o r X=H a n d R ( R = a l k y l s u b s t i t u e n t ) . The mechanism b y w h i c h t h e s e p r o d u c t s a r e f o r m e d was p r o p o s e d b y A l l i s o n a n d R i d g e i n 1 9 7 9 ( 2 ) , a n d i s shown i n r e a c t i o n (2) . The r e a c t a n t s f i r s t f o r m a c o m p l e x (a) , w h i c h may b e s i m p l y e l e c t r o s t a t i c a l l y bound. In (b) , t h e t r a n s i t i o n m e t a l i o n i n s e r t s i n t o t h e p o l a r C-X b o n d ( f o r m a l " o x i d a t i v e a d d i t i o n " ) , f o l l o w e d b y t h e shift of a H Co
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+ C H X 3
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C H X* · -Co 3
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- C H 3
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- Co
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- X
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(C H )Co (HX) 3
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CoC H 3
+ e
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+ HX
atom t h a t i s o n a C w h i c h i s β t o t h e m e t a l (0-H shift), across t h e m e t a l o n t o X, r e s u l t i n g i n t h e degradation o f t h e m o l e c u l e i n t o two s m a l l e r , s t a b l e s p e c i e s t h a t r e s i d e a s l i g a n d s on t h e m e t a l . The l a s t step (d) i s a c o m p e t i t i v e ligand loss. In this p r o c e s s , i t a p p e a r s t h a t t h e l i g a n d t h a t i s more w e a k l y bound t o t h e m e t a l i s p r e f e r e n t i a l l y l o s t (2) . I n some c a s e s , two p r o d u c t s r e s u l t from t h i s d i s s o c i a t i o n . F o r example, C o r e a c t s w i t h p r o p a n o l t o form b o t h CoH 0 and C o C H (2). W h i l e s t u d i e s o f s e r i e s o f , e.g., a l c o h o l s , a n d l a b e l i n g s t u d i e s p r o v i d e d some i n s i g h t s into t h e mechanisms that are operative i n the c h e m i s t r y , many i m p o r t a n t questions are d i f f i c u l t t o approach e x p e r i m e n t a l l y . To u n d e r s t a n d t h e c h e m i s t r y , i t i s c e r t a i n l y important t o understand t h e bonding o f the m o l e c u l e s and fragments i n v o l v e d t o t h e t r a n s i t i o n m e t a l and t h e s t r e n g t h s o f t h e s e bonds. Many f e a t u r e s such as t h e d i s t r i b u t i o n o f t h e charge i n t h e v a r i o u s i n t e r m e d i a t e s may a l s o be i m p o r t a n t i n c o n t r o l l i n g t h e t y p e s o f p r o d u c t s t h a t a r e formed. Thus, t h e o r e t i c a l studies are necessary t o understand the "driving f o r c e s " t h a t dominate t h e s e r e a c t i o n s and l e a d t o t h e r i c h chemistry. The c h e m i s t r y o f p o l a r compounds c o n t a i n i n g many t y p e s o f f u n c t i o n a l groups and m u l t i f u n c t i o n a l o r g a n i c compounds, w i t h b a r e t r a n s i t i o n m e t a l i o n s h a s b e e n +
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In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
18.
MAVRIDISETAL.
Gas-Phase Chemistry of Transition Metal Ions 265
reported. The r i c h e s t chemistry c e r t a i n l y involves t h o s e f u n c t i o n a l g r o u p s t h a t c o n t a i n n i t r o g e n , and a few e x a m p l e s w i l l be p r o v i d e d h e r e . S t e p n o w s k i and A l l i s o n ( A ) r e p o r t e d t h e chemistry of a v a r i e t y of t r a n s i t i o n metal ions with a s e r i e s of a l k y l cyanides. W h i l e i o n s s u c h as F e and C o insert i n t o t h e C-X b o n d i n many p o l a r compounds, t h e y do n o t a p p e a r t o do s o f o r -X = -CN. I n s t e a d , C-C b o n d s a r e c l e a v e d , a s shown f o r t h e C o / n - p r o p y l n i t r i l e system i n r e a c t i o n s ( 3 ) and ( 4 ) . +
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Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: June 25, 1990 | doi: 10.1021/bk-1990-0428.ch018
Co
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+ n-C H CN 3
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CoCH CN
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CoC H
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+ C H
s
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(3)
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(4)
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The r e a c t i v i t y o f n i t r i l e s may be d o m i n a t e d by a number o f a s p e c t s o f t h e c h e m i s t r y i n c l u d i n g t h e f a c t that C-CN bond dissociation energy (BDE) is s u b s t a n t i a l l y l a r g e r t h a n t h e BDE's f o r C - C l and C-OH bonds(4). A l s o , t h e r e i s evidence t h a t the geometry of t h e i n i t i a l c o m p l e x h a s a d r a m a t i c e f f e c t - an "end-on" i n t e r a c t i o n of the metal ions w i t h the n i t r i l e group may make i n s e r t i o n i n t o t h e C-CN bond g e o m e t r i c a l l y inaccessible(S). Another i n t e r e s t i n g f u n c t i o n a l group containing n i t r o g e n i s the n i t r o group. The c h e m i s t r y o f C o with a s e r i e s o f n i t r o a l k a n e s has b e e n r e p o r t e d by C a s s a d y et al.(£). Consider the Co / ~ N0 system. Fourteen d i f f e r e n t products were r e p o r t e d . Product i o n s s u c h a s C o ( C H ) and C o ( H N 0 ) s u g g e s t e d t h a t C o i n s e r t e d i n t o t h e C-N0 b o n d . Other product ions such as CoC H 0 and CoNO suggest that RN0 may be c o n v e r t e d , i n p a r t , t o R0- and NO- on t h e m e t a l c e n t e r . T h u s t h e -N0 group a c t i v e l y i n t e r a c t s w i t h the metal. The chemistry of organometallic anions with n i t r o a l k a n e s has a l s o been r e p o r t e d ( 7 ) . The c h e m i s t r y o f NO w i t h t r a n s i t i o n m e t a l i o n s h a s a l s o r e c e i v e d some a t t e n t i o n , and d e s e r v e s comment here. One o f t h e f i r s t ways i n w h i c h C o i o n s were g e n e r a t e d f o r mass s p e c t r o m e t r i c s t u d y was by e l e c t r o n i m p a c t i o n i z a t i o n o f t h e v o l a t i l e compound C o ( C O ) N O . In a d d i t i o n t o Co , i o n s s u c h a s CoCO and CoNO w e r e formed. W h i l e Co and C o C O i o n s f r e q u e n t l y r e a c t w i t h organic molecules in the gas phase, CoNO is unreactive(fi). The r e a s o n f o r t h i s c h a n g e i n c h e m i s t r y u p o n c h a n g e o f l i g a n d was u n c l e a r , s i n c e t h e r e was o n l y " t r a d i t i o n a l " bonding-schemes t o c o n s i d e r a t the time. However, t h e o r e t i c a l s t u d i e s show t h a t t h e CO l i g a n d i s n o t b o n d e d t o f i r s t row t r a n s i t i o n m e t a l i o n s v i a t h e D e w a r - C h a t t m o d e l , b u t i s e l e c t r o s t a t i c a l l y b o u n d (£,£) - t h u s i t ' s p r e s e n c e does not change t h e e l e c t r o n i c s t r u c t u r e of the metal. P r e s u m a b l y NO a c t s a s a 1 o r 3 - e l e c t r o n d o n o r w i t h Co* ( w h i c h h a s a 3 d g r o u n d s t a t e c o n f i g u r a t i o n ) , s o t h e CoNO i o n h a s a s i n g l e u n p a i r e d +
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In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
266
BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS
e l e c t r o n on t h e m e t a l , and s h o u l d n o t p a r t i c i p a t e i n r e a c t i o n s t h a t r e q u i r e an i n s e r t i o n s t e p . We a l s o n o t e t h a t t h e NO l i g a n d d o e s a p p e a r t o be r e a c t i v e when bound t o Co . J a c o b s o n r e p o r t e d the observation o f r e a c t i o n (5) i n v o l v i n g CO. +
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T h e r e h a v e b e e n a number o f s t u d i e s reported i n v o l v i n g m e t a l i o n r e a c t i o n s w i t h a m i n e s and ammonia. The f i r s t of these involved Co with a series of a m i n e s , r e p o r t e d by R a d e c k i and A l l i s o n i n 1 9 8 4 ( 1 1 ) . R e a c t i o n s (6-9) were observed f o r t h e Co /n-C H NH system·
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These r e s u l t s were c e r t a i n l y s u r p r i s i n g since insertion into the C-NH bond was not observed, a l t h o u g h t h e r e a c t i o n (10) 2
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should be exothermic. I t was proposed that H e l i m i n a t i o n o c c u r r e d by i n i t i a l i n s e r t i o n i n t o t h e N-H bond. While Co d i d n o t i n s e r t i n t o t h e C-NH b o n d o f p r i m a r y amines, i t does appear t o i n s e r t i n t o the C-N b o n d i n t h e t e r t i a r y a m i n e , ( C H ) N , and in allyl a m i n e . To e x p l a i n t h e f a i l u r e t o o b s e r v e r e a c t i o n (10) i t was p r o p o s e d t h a t t h e r e was a b a r r i e r along the r e a c t i o n channel - t h a t the i n s e r t i o n intermediate was energetically inaccessible for thermal energy r e a c t i o n s , w h i c h c o u l d o n l y be due t o a weak C o - N H bond. The BDE w o u l d h a v e t o be u n u s u a l l y s m a l l , < 20 kcal/mol. I t has s i n c e b e e n shown, a s d i s c u s s e d b e l o w , t h a t t h i s i s not the case. A n o t h e r e x p l a n a t i o n may be t h a t Co d o e s r e a c t w i t h , e.g., p r o p y l a m i n e , t o f o r m p r o p e n e and ammonia, b u t t h e r e i s a b a r r i e r t o the d i s s o c i a t i o n s t e p o f ( C H ) C o ( N H ) - w h i c h c a n n o t be d i s t i n g u i s h e d i n t h e mass s p e c t r u m f r o m t h e c o m p l e x containing the i n t a c t molecule, Co (C H NH ). The gas p h a s e c h e m i s t r y o f a number o f m e t a l i o n s w i t h a m i n e s has been s t u d i e d . B a b i n e c and A l l i s o n ( 1 2 . ) r e p o r t e d t h e chemistry o f C r , Mn , Fe , N i , or and Zn with η-propyl a m i n e . O n l y F e c l e a r l y i n s e r t e d i n t o the C-N bond, as e v i d e n c e d by the observation of the two products FeNH and F e C H . Mn (with a h a l f - f i l l e d d-shell, 3d 4s g r o u n d s t a t e ) and Z n (with a f i l l e d 2
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In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
18.
MAVRIDISETAL.
Gas-Phase Chemistry of Transition Metal Ions
1 0
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d-shell, 3d 4s ground s t a t e ) were u n r e a c t i v e with p r o p y l amine. In contrast, Cr (3d g r o u n d s t a t e ) and Cu (3d ground s t a t e ) were b o t h o b s e r v e d t o i n d u c e H elimination only. S i q s w o r t h and C a s t l e m a n ( 1 2 ) s t u d i e d the reactions of Ag " and Cu with methyl amine, d i m e t h y l a m i n e and t r i m e t h y l a m i n e . In these studies, h y d r i d e a b s t r a c t i o n was o b s e r v e d t o y i e l d t h e ( a m i n e H) i o n and t h e MH n e u t r a l p r o d u c t . I n 1987, B u c k n e r and F r e i s e r ( H ) showed t h a t Co d o e s f o r m a s t r o n g b o n d t o NH , and d i d s o a s f o l l o w s . I n an FTMS i n s t r u m e n t t h e y f o r m e d CoOH , w h i c h r e a c t s w i t h ammonia, r e a c t i o n ( 1 1 ) . +
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+
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(11)
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They also found that the NH group can be d i s p l a c e d f r o m Co* by b e n z e n e , b u t n o t by a c e t o n i t r i l e . W i t h t h i s i n f o r m a t i o n , t h e y c o n c l u d e d t h a t t h e BDE f o r Co -NH i s 65 ± 8 k c a l / m o l . I n t h i s same w o r k t h e y reported the chemistry of MNH w i t h some h y d r o c a r b o n s . They a l s o f o u n d t h a t F e O r e a c t s w i t h ammonia t o f o r m F e N H , and reported some c h e m i s t r y for this ionic species. T h e i r w o r k s u g g e s t e d t h a t t h e BDE f o r FeNH was > 41 k c a l / m o l and < 81 k c a l / m o l . T h i s w o r k was e x t e n d e d i n a r e p o r t by B u c k n e r , G o r d and F r e i s e r i n 1988 (IS). They f o u n d t h a t a v a r i e t y o f m e t a l ions including Sc , Ti and V r e a c t w i t h ammonia i n an e x o t h e r m i c p r o c e s s by r e a c t i o n ( 1 2 ) , w h i c h s u g g e s t s t h a t t h e s e M -NH b o n d s a r e s t r o n g e r t h a n 93 kcal/mol. The BDE f o r V -NH was d e t e r m i n e d t o be 101 ±7 k c a l / m o l , and t h e BDE f o r F e - N H t o be l o w e r , 54 ± 14 k c a l / m o l . 2
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They a l s o r e p o r t e d a number o f r e a c t i o n s f o r MNH w i t h neutral molecules. A v a r i e t y of mechanisms for r e a c t i o n (12) was d i s c u s s e d , i n c l u d i n g d e h y d r o g e n a t i o n v i a d i r e c t d e c o m p o s i t i o n o f t h e i n t e r m e d i a t e s (H) M -NH a n d / o r H-M -NH . 2
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The R e a c t i o n o f Sc+ w i t h NH Our i n t e n t i s t o g a i n t h e o r e t i c a l i n s i g h t s i n t o t h e i n i t i a l i n t e r a c t i o n of M with a p o l a r molecule, the insertion step and reductive elimination by a systematic study of the i n t e r a c t i o n of M w i t h NH . We begin here with S c b e c a u s e i t h a s o n l y two valence e l e c t r o n s and t h u s i t s t h e o r e t i c a l d e s c r i p t i o n and i t s c a l c u l a t e d i n t e r a c t i o n w i t h NH and i t s f r a g m e n t s w i l l be more r e l i a b l e than with the latter transition elements. We w i l l f i r s t d e s c r i b e t h e S c N core and then b u i l d up the various intermediates by adding hydrogens. S p e c i f i c f r a g m e n t s t o be s t u d i e d i n c l u d e ScNH , S c N H , HScNH and S c N H * . The c a l c u l a t i o n s w i l l i n c l u d e a l l e l e c t r o n s and w i l l u s e b o t h MCSCF and 3
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In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
268
BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS
CI t e c h n i q u e s . The b a s i s s e t s used a n d the approach have been d e s c r i b e d previously(16).
general
+
S£H . The r e l a t i v e e n e r g i e s o f t h e l o w l y i n g s t a t e s o f S c , Ν, NH and N H a r e c o l l e c t e d i n F i g u r e 1. B e c a u s e t h e e x c i t e d s t a t e s o f Ν a r e much h i g h e r i n e n e r g y t h a n t h o s e o f S c we w i l l c o n s i d e r t h e s t a t e s o f ScN which c o r r e l a t e t o the ground s t a t e o f Ν and the s d and d configuration of Sc (1£). As S c i n an s d c o n f i g u r a t i o n a p p r o a c h e s Ν we c a n i m a g i n e f o r m i n g a σ bond b e t w e e n t h e 4 s e l e c t r o n o n S c and a 2p σ e l e c t r o n on Ν and a π bond b e t w e e n t h e 3 d and 2 ρ e l e c t r o n s . This leaves an unpaired e l e c t r o n on Ν i n t h e 2 p o r b i t a l and r e s u l t s i n a Π s t a t e r e p r e s e n t e d b y 2
+
2
+
+
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+
x z
χ
2
I t i s o n l y a t l a r g e ScN s e p a r a t i o n s t h a t the Sc e l e c t r o n i n t h e a bond i s p u r e 4 s . A s t h e bond f o r m s o t h e r o r b i t a l s o n S c o f a symmetry mix i n t o t h e b o n d i n g orbital. T h i s i s s e e n most v i v i d l y i n F i g u r e 2 a w h i c h shows t h e o c c u p a n c i e s o f v a r i o u s a t o m i c o r b i t a l s i n t h e Π state o f ScN as a function o f separation. At e q u i l i b r i u m t h e a t o m i c o r b i t a l s i n t h e σ bond have t h e occupancy 2
+
(4s°-° 4 p / - °
3d
a
°-
e e
)
(2
S c
l P < 7
-
2 8
)
N
and t h e 4 s o r b i t a l h a s gone f r o m a n o c c u p a n c y o f 1 t o 0. I n t e r e s t i n g l y t h e π bond occupancy i s e q u a l t o t h a t o f t h e a bond < xz°- >Sc< Px ' >N 3d
ei
l
2
while t h e unpaired l o c a l i z e d o n N.
2e
electron
remains
essentially
Pdy^-^JscUPy *' )» 0
1
+
The n e t r e s u l t i s t h a t S c l o o s e s 0.49 e l e c t r o n s t o Ν r e s u l t i n g i n the charge d i s t r i b u t i o n + 1
*
6 1
Sc
Ν
- 0
·
4 β
.
I f we b r e a k t h e η bond i n t h i s s t a t e and p l a c e t h e h i g h s p i n e l e c t r o n o n S c i n a d i o r b i t a l we w i l l f o r m a Δ^. s t a t e . +
4
In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
y
18.
Gas-Phase Chemistry of Transition Metal Ions 269
MAVRIDIS ET A L
60
2p ( D) 3
2
(55)
50 _ 40 ο
(36)
σ π 2
2
(*Δ)
σπ
2
( A ) 2
t
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s
(32)
^ 30 « ο UJ 20 <
τι
10
(69)
2p ( S) 3
σ π2( Σ~)
4
2
3d (3 ) 2
_4s3d(lD)_
σ π( Β!)
3
2
NH
F
4s3d( D)
2
3
Sc
NH1
F i g u r e 1. R e l a t i v e E n e r g y L e v e l s ( k c a l / m o l " ) NH(20), NH (21) and S c ( 1 2 ) .
1
o f N(i£),
+
2
1.4-
1.4·
a
Ν p >J/ x
1.2H
1.2·
l\ Ι
1.0-
1.0
'
I i
Ν Py^V|'7
c ο ο
0.8·
D Q. Ο CL
0.6·
.^Sc
4s
+
/-Se*
Ν ρ ^'
0.8
σ
ό
Ή
ν/ 0.6 Tronafer
0.4
| \
0.4-
d
ScN + Π 0.2-
'
0.0
0.0 0.0
2.0
| \
2
\ 0.2 • Sc+
Chorge Transfer
4.0
6.0
8.0
10.0
R(Sc-N) (au) F i g u r e 2. O r b i t a l occupancy internuclear distance and b) Σ states. 2
0.0
Sc + d,
2.0
4.0
6.0
8.0
10.0
R(Sc-N) (au) as a f o r ScN
+
function of i n t h e a) Π
+
In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
2
270
BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS
+
I n a s i m i l a r way we c a n c o n s i d e r s t a t e s o f S c N which c o r r e l a t e t o the d (excited) state of S c . For example t h e Σ s t a t e h a s a π , * d o u b l e bond a n d a n u n p a i r e d e l e c t r o n on Ν i n t h e σ system 2
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2
+
+
[ *v* Γ I f we b r e a k a π bond i n t h i s s t r u c t u r e a n d p l a c e t h e t h e n u n c o u p l e d d* e l e c t r o n i n a do* o r b i t a l we h a v e S c N i n a Φ s t a t e h e l d t o g e t h e r by a s i n g l e η bond ±
+
4
C o n f i g u r a t i o n i n t e r a c t i o n c a l c u l a t i o n s on t h e s e four s t a t e s r e s u l t i n t h e p o t e n t i a l c u r v e s shown i n F i g u r e 3. The o r b i t a l p o p u l a t i o n s i n t h e Σ s t a t e a r e shown i n F i g u r e 2b a n d t h e p r o p e r t i e s o f a l l f o u r s t a t e s a r e summarized i n T a b l e I . 2
Table I .
State 2
2
+
Calculated Properties of Several S t a t e s o f ScN*
Lewis 2
ScZN^ ° π
n (sd)
* π */ Se r Ν σ
x
4
A(sd)
4
Φ(ά )
ω^,ίοιη" ) O f S c )
D (kcal/molϊ
R fÀ)
63.0
1.738
871
+ 1.51
55.3
1.804
811
+ 1.51
26.8
2.101
534
+1.46
21.1
1.979
674
+1.47
c
Z (d )
+
c
1
Ρχ
2
N o t e t h a t a l l D 's a r e r e p o r t e d r e l a t i v e t o t h e g r o u n d s t a t e s o f S c ( D ) and N ( S ) . Most i n t e r e s t i n g l y , t h e g r o u n d s t a t e i s t h e Σ w h i c h c o n t a i n s two π b o n d s a n d which c o r r e l a t e s w i t h t h e e x c i t e d d configuration of +
3
4
2
+
2
In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Gas-Phase Chemistry of Transition Metal Ions 271
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18. MAVRIDIS ET AL.
In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
272
BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS
Sc . I t i s a l s o n o t e w o r t h y t h a t t h e c h a r g e on S c , Q, i s i n d e p e n d e n t o f t h e number a n d t y p e o f f o r m a l b o n d s i n the molecule. ScNH+ We c a n i m a g i n e S c N H b e i n g f o r m e d b y a d d i t i o n o f a H t o either the Σ or n state of ScN . +
2
+
2
+
2
Sc » Ν — Η *
2
Sc - Η · * [ Σ * ] • · Η [ 5 ] Ρ
or Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: June 25, 1990 | doi: 10.1021/bk-1990-0428.ch018
1
J Σ*
Η Se » Ν * [
2
2
Π ] • ·Η [ S J
O p t i m i z i n g t h e s t r u c t u r e o f t h i s system a t t h e CI l e v e l r e s u l t s i n t h e l i n e a r s t r u c t u r e w i t h a ScN-H d i s s o c i a t i o n e n e r g y o f 118.5 k c a l / m o l ( K u n z e , K.L. and H a r r i s o n , J . F . , MSU, u n p u b l i s h e d results). This i s a r e m a r k a b l y s t r o n g NH b o n d ( f r e e NH h a s a o f 80 k c a l / m o l and we r e q u i r e 108 k c a l / m o l t o b r e a k t n e f i r s t N-H b o n d i n N H ) . The r e a s o n f o r t h i s e n h a n c e d NH b o n d s t r e n g t h i s a p p a r e n t when one a n a l y z e s t h e e l e c t r o n d i s t r i b u t i o n i n t h e S c N H m o l e c u l e and c o m p a r e s i t w i t h S c N and NH. T h i s a n a l y s i s shows t h a t when H b o n d s t o S c N t h e Ν 2 s o r b i t a l l o s e s 0.45 e l e c t r o n s , t h e Η I s l o s e s 0.17e a n d t h e Ν 2ρ o r b i t a l gains 0.30 electrons. If, for b o o k k e e p i n g p u r p o s e s , we assume a l l o f t h e e l e c t r o n s l o s t b y Η go i n t o t h e Ν 2 ρ and t h a t t h e r e m a i n i n g g a i n i n t h e Ν 2 ρ comes f r o m t h e Ν 2 s , t h e n we c a n a l l o t t h e remaining 0.32 e l e c t r o n s l o s t b y t h e Ν 2 s t o t h e σ o r b i t a l s on S c . T h i s a c c o u n t s n i c e l y f o r t h e i n c r e a s e o f 0.33 e l e c t r o n s i n t h e S c σ s y s t e m and s u g g e s t s t h a t t h e e n h a n c e d N-H b o n d s t r e n g t h i n S c N H i s due t o a s i g n i f i c a n t d a t i v e bond formed between Sc and t h e Ν 2s electron pair. A l t e r n a t i v e l y , we may i m a g i n e forming ScNH v i a the lowest s t a t e ( F) of the e x c i t e d d conf i g u r a t i o n . +
3
+
+
+
σ
α
σ
+
+
+
+
3
Sc
+
3
3
( F ) + NH
( Σ~)
2
- ScNH
+
+
(*Σ )
+
The S c - NH b o n d s t r e n g t h i s c a l c u l a t e d t o be 105.9 k c a l / m o l r e l a t i v e t o t h e g r o u n d s t a t e f r a g m e n t s ( K u n z e , K.L. and H a r r i s o n , J . F . , MSU, unpublished r e s u l t s ) , w h i c h s h o u l d be compared w i t h r e c e n t r e s u l t s b y A r m e n t r o u t e t a l . ( A r m e n t r o u t , P., U n i v . o f U t a h , p r i v a t e c o m m u n i c a t i o n , 1989) s u g g e s t i n g a S c - N H b o n d s t r e n g t h o f 118 k c a l / m o l . T h i s Sc -NH c a l c u l a t e d bond s t r e n g t h i s 43 k c a l / m o l s t r o n g e r t h a n t h e 63 k c a l / m o l c a l c u l a t e d f o r t h e S c N bond i n t h e Σ state. Note t h a t t h e c a l c u l a t e d N-H b o n d s t r e n g t h i n S c N - H i s a l s o 43 k c a l / m o l s t r o n g e r t h a n t h e 75.5 k c a l / m o l c a l c u l a t e d +
+
+
2
+
+
In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Gas-Phase Chemistry of Transition-Metal Ions 273
18. MAVRIDIS ET A L
bond s t r e n g t h i n NH. This s y n e r g i s t i c r e l a t i o n s h i p between the N-H and Sc-N bonds in ScNH is a c o n s e q u e n c e o f t h e f r e e d o m g i v e n t o t h e a s y s t e m by t h e u n u s u a l π, π d o u b l e b o n d b e t w e e n t h e m e t a l and N. As Η b o n d s t o t h e ρσ e l e c t r o n on Ν c o n s i d e r a b l e 2s c h a r a c t e r is mixed into the NH bonding orbital. This h y b r i d i z a t i o n n o t o n l y s t r e n g t h e n s t h e N-H b o n d b u t t h e companion orbital which points toward the Sc simultaneously s t a b i l i z e s t h e ScN b o n d by a dative interaction. The Σ s t a t e r e p r e s e n t e d a b o v e i s more a c c u r a t e l y w r i t t e n as +
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2
+
+
Sc
1
«- N-H
+
(Σ)
+
ScNH The g r o u n d s t a t e o f NH i s of B symmetry and is c h a r a c t e r i z e d by a σ π e l e c t r o n i c c o n f i g u r a t i o n and an NH a n g l e o f 104°, w h e r e a s , t h e f i r s t e x c i t e d s t a t e i s of A symmetry and i s 32 k c a l / m o l a b o v e t h e g r o u n d B . It i s characterized by a σ* electronic c o n f i g u r a t i o n and an a n g l e o f 144° 2
2
2
2
X
ι
2
2
X
2
1
2
X
We may i m a g i n e t h e g r o u n d s t a t e o f NH reacting w i t h e i t h e r the ground or e x c i t e d s t a t e of S c t o form a doublet with a nonplanar s t r u c t u r e i n which the b o n d i n g e l e c t r o n on S c i s f o r m a l l y e i t h e r 4s o r 3άσ. A l t e r n a t i v e l y we c a n i m a g i n e t h e g r o u n d s t a t e o f NH r e a c t i n g w i t h the 3d c o n f i g u r a t i o n of S c t o form t h e p l a n a r m o l e c u l e h a v i n g a π b o n d and a ( d a t i v e ) σ b o n d . The u n p a i r e d e l e c t r o n on Sc w o u l d most l i k e l y be in a S o r b i t a l t o o p t i m i z e t h e a b i l i t y t o f o r m a π and d a t i v e " a bond. 2
+
+
2
+
2
±
J . F . , MSU, u n p u b l i s h e d work) p r e d i c t t h a t t h e s t a t e of ScNH i s of A symmetry and that p l a n a r w i t h the geometry +
2
2
2
[
•SC«N^ >07°1 H
1 9 β /
ι.οΓ
Η
J
In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
ground i t is
274
BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS
Only s m a l l e n e r g i e s a r e r e q u i r e d t o d e v i a t e from the planar structure. Our c a l c u l a t i o n s s u g g e s t t h a t 0.3 k c a l / m o l i s r e q u i r e d t o move S c 10* o u t o f p l a n e and 0.9 k c a l / m o l t o move i t 2 0 * . The e l e c t r o n d i s t r i b u t i o n a t e q u i l i b r i u m s u g g e s t s t h a t S c h a s a t o t a l c h a r g e o f +1.51 h a v i n g l o s t 0.49 e l e c t r o n s t o t h e NH g r o u p . The e l e c t r o n s r e m a i n i n g o n Sc a r e d i s t r i b u t e d a s f o l l o w s 2
4
s
0
.0 6
4
0.0
p
4
p
0 .0 7
3
0 .1 e
d
3
(
j
0 .22
3
(
j
1 .0 0
+
We c a l c u l a t e t h e S c = N H b o n d d i s s o c i a t i o n e n e r g y f o r t h e A ^ s t a t e t o be 79 k c a l / m o l w h i c h c o m p a r e s f a v o r a b l y w i t h t h e 85 k c a l / m o l r e c e n t l y d e t e r m i n e d b y Armentrout e t a l . (Armentrout, P.Β., U n i v . o f U t a h , p r i v a t e communication, 1989). The l o w e s t A s t a t e has the unpaired electron in a 5+ orbital and i s e s s e n t i a l l y degenerate w i t h t h e ground A state. The lowest B s t a t e has a D o f 70 k c a l / m o l a n d r e s u l t s when t h e u n p a i r e d electron i s i n a d* orbital. Placing the unpaired e l e c t r o n i n a d * o r b i t a l r e s u l t s in the B s t a t e w h i c h i s 22 k c a l / m o l a b o v e t h e A ground state. We have a l s o i n v e s t i g a t e d s e v e r a l quartet s t a t e s which a r e bound b y -30 kcal/mol, p r i m a r i l y by c h a r g e - d i p o l e i n t e r a c t i o n s .
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2
2
2
l
2
2
2
2
e
y
x
2
2
X
2
SçHH»* Three isomers w i t h t h i s formula a r e r e l e v a n t . The first i s a c h a r g e - d i p o l e c o m p l e x i n w h i c h NH is intact. T h i s c o m p l e x i s bound r e l a t i v e t o g r o u n d s t a t e f r a g m e n t s b y 36.8 k c a l / m o l . The second isomer i s the insertion product H-Sc-NH . G e n e r a l i z e d V a l e n c e Bond c a l c u l a t i o n s o n t h i s s y s t e m s u g g e s t t h a t i t i s bound r e l a t i v e t o S c + NH b y 16 k c a l / m o l . The g e o m e t r y i s 3
2
+
3
The third isomer i s the electrostatic complex ( H ) S c N H t h a t i s b o u n d r e l a t i v e t o S c + NH b y 14 kcal/mol. Interestingly the H m o l e c u l e i s bound t o the ScNH i o n by 5 k c a l / m o l . +
+
2
3
2
+
Summary o f E n e r g e t i c s The r e l a t i v e e n e r g i e s o f t h e v a r i o u s p r o d u c t s o f t h e r e a c t i o n o f S c ( D ) w i t h NH are collected i n Figure 4. T h i s f i g u r e was c o n s t r u c t e d u s i n g t h e e n e r g i e s discussed i n the previous s e c t i o n as w e l l as b i n d i n g energies of ScH and ScH (l£) reported earlier. Energies determined experimentally are shown i n parentheses. +
3
3
+
+
2
In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Gas-Phase Chemistry of Transition-Metal Ions
MAVRIDIS ET AL.
S c ( D )+NH +
3
3
> Products
ν
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•277 Se"* Ν • 3Η· 1
+214 ScN*+3H*
+201 Sc* + ΝΗ+2Η·
106(119) 104 (115) • 108 Sc + ΝΗ +Η· +
2
+97 ScH^+NH
•95 ScNH*+2H*
78(85) ScH*+NH
2
|_
-37 Sc"** NH (ion-dipole) 4
3
F i g u r e 4. Summary o f E n e r g e t i c s f o r S c
+
+ NH .
In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
3
276
BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS
These c a l c u l a t i o n s suggest t h a t b o t h t h e i n s e r t i o n product H-Sc-NH and t h e complex H ScNH are exothermic products o f t h e r e a c t i o n o f Sc w i t h NH . The i n s e r t i o n p r o d u c t i s f o r m e d e x o t h e r m i c a l l y b e c a u s e t h e Sc-NH bond s t r e n g t h a n d t h e Sc-H bond s t r e n g t h a r e l a r g e a n d a b l e t o o v e r c o m e t h e 108 k c a l / m o l r e q u i r e d t o b r e a k a N-H b o n d i n N H . O u r c a l c u l a t i o n s s u g g e s t t h a t t h e S c - N H bond i s s t r o n g because: 1. The NH group donates charge t o t h e t r a n s i t i o n metal i o n v i a t h e σ p a i r o n N. This dative i n t e r a c t i o n i s o p t i m i z e d when t h e r e a r e n o 4 s o r 3da e l e c t r o n s o n t h e m e t a l . 2. The m e t a l d o n a t e s c h a r g e t o t h e π o r b i t a l o f t h e NH g r o u p . T h i s t r a n s f e r i s o p t i m i z e d when t h e r e i s o n e d* e l e c t r o n i n t h e * s y s t e m b e i n g coupled w i t h t h e NH π o r b i t a l . I n t e r f e r i n g w i t h e i t h e r o f t h e s e mechanisms w i l l reduce t h e m e t a l - NH bond s t r e n g t h a n d c o n s e q u e n t l y t h e p o t e n t i a l f o r a n e x o t h e r m i c H-M-NH insertion product. L i k e w i s e r e d u c i n g t h e metal-H bond s t r e n g t h would a l s o reduce t h e p o s s i b i l i t y o f an exothermic i n s e r t i o n product. F o r e x a m p l e , Co"" i s a d s y s t e m a n d r e g a r d l e s s o f the c o n f i g u r a t i o n o f t h e e l e c t r o n s i n t h e ground F s t a t e one w i l l a l w a y s have a t l e a s t one e l e c t r o n , i n a da o r b i t a l . Consequently t h e planar c o n f i g u r a t i o n i s n o t o b v i o u s l y b e t t e r t h a n t h e s i n g l y bonded non p l a n a r structure +
# e ,
2
+
2
3
2
3
+
2
?
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2
2
2
2
+
2
1
e
3
and, a s a r e s u l t , t h e M-NH b o n d s t r e n g t h w i l l b e s m a l l e r f o r Co t h a n S c . S o w h i l e S c a n d C o * b o t h h a v e two singly occupied d orbitals Co i s less e n e r g e t i c a l l y f a v o r e d t o r e a c t w i t h NH t o f o r m 2
+
+
3
+
H - Co - N H . 2
S i m i l a r considerations apply t o t h e exothermic products H + ScNH . I n t h i s case t h e e x o t h e r m i c i t y r e s u l t s f r o m t h e s t r o n g M-NH b o n d w h i c h a r i s e s f r o m : 1. Each 2ρ e l e c t r o n on Ν bonding t o a singly occupied o r b i t a l on S c * and 2. T h e l o n e p a i r o n Ν b o n d i n g t o t h e empty 4 s , 3 d o r b i t a l s on S c . +
2
π
In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
18. MAVRIDIS ET AL.
Gas-Phase Chemistry of Transition-Metal Ions
277
+
I t i s not possible f o r C o t o s a t i s f y both o f these c o n d i t i o n s s i m u l t a n e o u s l y and consequently, CoNH s h o u l d h a v e a r e l a t i v e l y weak M e t a l - N H b o n d s t r e n g t h . The p r o c e s s +
Co
+
+ NH
3
- Co=NH
+
+ H
2
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w i l l b e l e s s e x o t h e r m i c i f t h e Co=NH l e s s t h a n 90 k c a l / m o l .
+
bond s t r e n g t h i s
Acknowledgments T h i s w o r k was p a r t i a l l y s u p p o r t e d u n d e r NSF G r a n t s CHE8519752 (JFH) a n d CHE8722111 ( J A ) , a s w e l l a s a NATO G r a n t CRG890502 t o A. M a v r i d i s a n d J . F. H a r r i s o n . The E l e c t r o n S t r u c t u r e codes p r o v i d e d by t h e Argonne Theory g r o u p h a v e b e e n i n d i s p e n s a b l e t o t h i s work.
Literature Cited 1. Allison, J . Prog. Inorg. Chem. vol. 34, S.J. Lippard, Ed. 1986, p. 627. 2. Allison, J.; Ridge, D.P. J . Am. Chem. Soc. 1979, 101, 1332. 3. Tsarbopoulos, Α.; Allison, J . Organometallics 1984, 3, 86. 4. Stepnowski, R.M.; Allison, J . Organometallics 1988, 7, 2097. 5. An end-on interaction of the metal ion with the CN group results in reactions that are remote from the site of complexation. Such reactions are also discussed in: Tsarbopoulos, A.; Allison, J . J . Am. Chem. Soc. 1985, 107, 5085. 6. Cassady, C . J . ; Freiser, B.S.; McElvany, S.W.; Allison, J . J . Am. Chem. Soc. 1984, 106, 6125. 7. McElvany, S.W.; Allison, J . Organometallics 1986, 5, 1219. 8. Allison, J . ; Mavridis, A.; Harrison, J.F. Polyhedron, 1988, 16/17, 1559. 9. Mavridis, A.; Harrison, J . F . ; Allison, J . J . Am. Chem. Soc. 1989, 111, 2482. 10. Jacobson, D.B.; J. Am. Chem. Soc. 1987, 109, 6851. 11. Radecki, B.D.; Allison, J . J . Am. Chem. Soc. 1984, 106, 946. 12. Babinec, S . J . ; Allison, J . J . Am. Chem. Soc. 1984, 106, 7718. 13. Sigsworth, S.W.; Castleman, J r . , A.W. J. Am. Chem. Soc. 1989, 111, 3566. 14. Buckner, S.W.; Freiser, B.S. J . Am. Chem. Soc. 1987, 109, 4715. 15. Buckner, S.W.; Gord, J.R.; Freiser, B.S. J . Am. Chem. Soc. 1988, 110, 6606. 16. Kunze, K.L. and Harrison, J . F . J . Phys. Chem. 1989, 93, 2983. 17. Alvarado Swaisgood, A.E. and Harrison, J . F . J . Phys. Chem. 1985, 89, 5198.
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18. 19. 20. 21.
BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS
Sunderlin, L.; A r i s t o v , Ν and Armentrout P . B . J. Am. Chem. Soc. 1987, 109, 78 Moore, C. E . , "Atomic Energy Levels; Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. C i r c u l a r 35, Washington, DC, 1971, V o l . I and II. Huber, K . P . and Herzberg, G . , "Moleculear Spectra and Molecular Structure", Van Nostrand Reinhold Co., New York, 1979. Johns, J.W.C.; Ramsay, D.A. and Ross, S . C . , Can. J. Phys., 1976, 54, 1804. December 19, 1989
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RECEIVED
In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
